1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Brief Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (IC's). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In particular in a lithographic apparatus using a projection beam of radiation in the extreme ultraviolet (EUV) range or a particle beam, it is necessary to operate in high vacuum in order to prevent undesirable phenomena, like for instance absorption and/or scattering of the beam of radiation as a result of the presence of gas molecules in the lithographic apparatus, and deposition of contaminants onto surfaces of optical devices. Thus, in such lithographic apparatus as presently known in the art, the substrate and/or patterning devices are located in a vacuum chamber in which, in operation, a high vacuum is maintained. As a consequence, in a practical embodiment of a lithographic apparatus, a support structure for the substrate and/or a support structure for the patterning device, or a number of such support structures in case of a multiple stage apparatus, as well as positioning structures associated with the support structures, will be located in a vacuum chamber.
For the sake of completeness it is noted that in this specification the expression “high vacuum” indicates a very low pressure. Similarly the expression “low vacuum” indicates a relatively high but still sub-atmospheric pressure.
Use of a high vacuum imposes rather severe requirements on the components that are put into the vacuum. Such components should be constructed from materials which in vacuum will feature no, or only a minimum of, outgassing of contaminants. In this connection the expression “outgassing” includes both outgassing from the materials as such and from gases adsorbed on their surfaces.
The support structures for the patterning device and the substrate in a lithographic apparatus are of a rather complicated design, employing various auxiliary elements like, for instance, sensors, sensor electronics, positioning structures, electric wiring and conduits for cooling purposes and the like. For all such elements, when used in a high vacuum, special materials may be used in order to minimize the outgassing problem indicated above. The necessity of using such elements, especially designed for a high vacuum environment, does not only result in high costs in connection with manufacturing and/or purchasing and maintaining a working stock of such elements, but also limits the freedom of design for such elements.